1. Field of the Invention
The present invention relates generally to a method for reclaiming used foundry sands and, specifically, to a method for controlling the oxidation of residual organic binders and calcination of residual inorganic binders adhering to the surface of sand grains in waste foundry sands.
2. Description of the Prior Art
Foundry sand is commonly used to make core molds into which ferrous and non-ferrous metals are cast. The core molds consist of sand bonded with special additives including inorganic binders such as clay and organic binders, such as phenols, melamine, or urea formaldehyde.
Previously, after the casting had set within the mold, the mold was broken away and discarded. Various factors such as the depletion of natural sand deposits and the cost of disposing of used sand in accordance with recent environmental regulations have now made it economical and advantageous to reclaim the used sand for repeated use.
The type binder used has, in the past, generally dictated the type of reclamation process utilized. Thus, for the inorganic, clay-bonded sands, so-called "wet" and "dry" scrubbing techniques have been employed. U.S. Pat. No. 2,261,947, to Barnebl et al., issued Nov. 11, 1941, entitled Foundry Practice, utilizes the wet scrubbing method in which clay-bonded sands are cleaned in a blasting room by means of high pressure streams of water and sand projected from guns. In the "dry" scrubbing technique, the "dry" sand is projected against an abrasive surface to crack off the clay binder. In both cases, the cleaning action is that of mechanical attrition.
For sands utilizing organic resin binders, thermal reclamation techniques have commonly been employed. For example, U.S. Pat. No. 2,478,461, to Connolly, issued Aug. 9, 1949, entitled Apparatus and Method for Treating Foundry Sand, discusses the reclaiming of foundry sand by heating or roasting treatment which causes the organic binders to be burned away. The prior art thermal reclamation techniques have included the use of multiple-hearth type furnaces with mechanical "rabble arms" which worked over the used sand and moved the sand through the hearth at elevated temperatures.
None of the previously described techniques proved to be entirely satisfactory for use in reclaiming both the clay-bonded and organic resin-bonded sands, as where a mixture of these sands is present. Consistency of final product was also a problem with prior art systems. Also, the efficiency of the previous techniques do not exceed about 70%, with the balance of the waste sand being discarded after each reclamation cycle into municipal landfills.
U.S. Pat. No. 4,549,698, issued Oct. 29, 1985, to Andrews, entitled Method of Reclaiming Foundry Sand, and assigned to the assignee of the present invention, presented a novel method for reclaiming mixtures of both organic resin-bonded sands and clay-bonded sands. The method utilized a triple fluid bed thermal reactor which combined thermal and abrasive action within the beds to remove both resin-bonded and clay-bonded materials in one continuous operation.
U.S. Pat. No. 4,978,076, issued Dec. 18, 1990, to Andrews and Reier, entitled Method for Separating Hazardous Substances in Waste Foundry Sands and assigned to the assignee of the present invention, presented further improvements in techniques for separating silica sand grains from layers of organic and inorganic binders adhering to the surface of the sand grains so as to reclaim the sand for reuse in foundry manufacturing processes. The technique was especially adapted for separation of hazardous components, such as lead, from the sand aggregates.
The present invention is directed to further improvements in thermal sand reclamation techniques by providing precise control of the temperature and oxygen content in the fluidizing gases used in the previously described reactor fluid beds, particularly in the calcination fluid bed. It is this stage in the reclamation process that determines whether the inorganic binders are separated from or fused onto the used sand grain surfaces. While other thermal reclamation techniques have been utilized in the prior art in an attempt to reclaim waste foundry sand utilizing organic resin binders, those techniques employing fluid bed technology generally inject and burn any supplemental fuel gas required to heat the sand bed in and into the sand bed itself. Precise temperature control of such a random fuel mixture was difficult or impossible to achieve and extreme unevenness of heat within the bed resulted. As a result, the physical and chemical properties of the reclaimed sand were found to vary so that the physical properties of the resulting molds and cores varied as well.
Another problem which existed in those prior art processes where inorganic binders were mixed with organic binders for thermal reclamation by burning in a fluid bed, and where supplementary gas or fossil fuel was introduced over the surface of or into the fluidized bed, was the creation of "hot spots" within the calcining fluid bed due to random combustion. The result was a random "fusing" of inorganic binders to the sand grain surfaces, rendering them unsuitable for making molds and cores by rebonding with resin binders. As a result, those fluid bed units employing direct combustion of fuel gas and resin binders in the fluid bed have not proved to be successful in practice, particularly when operated on a mixture of inorganic and organic waste sands to thermally reclaim the sand for reuse in making molds and cores rebonded with acid catalyzed resin binders.
The present invention has as its object to provide a method for controlling the rate of oxidation and calcination of a random mixture of organic and inorganic bonded waste foundry sands to ensure consistent physical and chemical sand properties when reused with organic binders.
A further object of the invention is to maximize the desirable physical properties of the rebonded molds and cores, allowing the use of the same or less resin binder than with new sand, as a result of consistent sand qualities very similar and equal to the qualities of new sand.